Reactions of 9-Substituted Fluorenide Carbanions with Allyl Chlorides by SN2 and SN2' Mechanism

Article abstract of DOI:10.1021/ja00240a029

The average ratio of S_N2 rate constants for benzyl chloride vs. allyl chloride reacting with 9-substituted fluorenide carbanions, 9-G-Fl^- (G = Me, PhS, Ph o-tol, or t-Bu), in Me2SO solution is 2.3, which is close to the ratio previously observed for reactions of smaller nucleophiles with more localized charges, such as iodide ion, in various solvents.Broensted plots of log k vs. pK_HA (both in Me2SO) are used to calculate reactivity values (r) for 9-G-Fl^- ions of the same basicity reacting with allyl chloride.The r values reveal the same order of steric effects for G in these S_N2 reactions as was found previously for benzyl chloride, i.e., Me < Ph < o-tol < t-Bu, but the effects are slightly smaller for allyl chloride.The second-order rate constants for reactions with four families of 9-G-Fl^- ions were found to increase progressively and appreciably with γ-methyl substitution, i.e., CH2=CHCH2Cl (1) , MeCH=CHCH2Cl (4) , Me2C=CHCH2Cl (5), whereas the Broensted β values decreased progressively.The second-order rate constants for reactions with families of 9-G-Fl^- ions were found to follow a somewhat different pattern for α-methyl substitution, i.e., CH2=CHCH2Cl (1) > CH2=CHCH(Me)Cl (2) <*> CH2=CHC(Me)2Cl (3); the rate differences were small, but the β values again decreased progressively.Product studies indicated that 1 reacts with 9-G-Fl^- ions by an S_N2 mechanism but that 2 gives S_N2' products in a amounts increasing along the series G = Me, Ph, o-tol, t-Bu; 3 appears to give only S_N2' products.The rates of reactions of 9-CN-Fl^- and 9-CO2MeFl^- ions with 3 to form S_N2' and S_N2 products are independent of the carbanion concentration.The first-order reaction of 9-CN-Fl^- ion with t-BuCl in MeOH gives 17percent yield of 9-CN-9-t-BuFl.Entalpies of activation for S_N2 reactions of 9-G-Fl^- ions reacting with 5 are 7-9 kcal/mol lower, and entropies of activation are 18, or more, eu units less positive than for S_N2' reactions of 9-G-Fl^- ions with 3.Mechanism for S_N2' reactions are discussed in light of these findings.